Experiments described in this proposal will investigate the cellular constituents and arrangement of motor pattern generating circuits in the spinal cord. Recently, general organizing principles have been established for spinal circuitry. However, further dissection of these circuits is hampered by a lack of markers for precise functional classes of neurons, and technological obstacles in recording the activity of large numbers of neurons. Proposed experiments will test the hypothesis that evolutionarily conserved elements in transcription factor promoters are sufficient drive gene expression in functionally defined neuronal populations. A combination of genetic and optical approaches will allow the identification of novel neuron types and their functional characterization in an intact, functioning neural network. Overall, these experiments will provide a detailed spatial-temporal picture of how spinal systems coordinate movements.